Recent years have seen an ever-increasing interest in the application of novel materials in the medical and pharmaceutical fields, whether as prostheses or in medical devices designed for contact with biological environment of the living body. Of these materials, polymers, mainly synthetic polymers, are by far the most diverse classes that are found to impart considerable benefits to the patient health care.
The applications of polymers in the medical and pharmaceutical field are wide ranging. In the medical field, polymers are employed as implants or support materials such as artificial organs, vascular grafts, intraocular lenses, artificial joints, mammary prostheses, suture materials, extracorporeal therapeutics or other support materials such as those used in hemoperfusion, blood oxygenators, catheters, blood tubing, wound and burn covering materials, splints, contact lenses etc. In the pharmaceutical field, polymers have been particularly used in development of nanoparticle delivery systems and controlled-release delivery systems. Extensive studies are also being pursued to target drugs with delivery systems to the desired site. Further, polymers have found great utility in other applications as well, such as transdermal drug-delivery patches, micro spheres, bioprocesses such as enzyme and cell immobilization etc.
Amongst such applications, nanoparticluate drug delivery systems have been more extensively studied and nanometer-size drug carriers with hydrophilic surfaces, specially those comprising two spherical co-centric regions of polymeric micelles—a densely packed core of a hydrophobic material, which is responsible for entrapping a hydrophobic drug or compound and an outer shell made of hydrophilic material have been extensively studied. Such systems are found to evade recognition and uptake by the reticulo-endothelial systems (RES) and thus can circulate in the blood for a long time. Further, due to their extremely small size (a polymeric micelle usually consists of several hundred block copolymers and has a diameter of about 20 nm-50 nm), the particles extravasate at the pathological sites, such as solid tumors through passive targeting mechanism.
Polymers derive their range of properties attributable to their chemical and structural features. The polymer chains may essentially be linear, branched, or cross-linked to adjacent chains. Furthermore, these chains may be unordered, ordered or oriented in a single direction. These structural features combined with the chemical composition, lends a variety of properties to polymers, resulting in a variety of end-use applications. Further, these structural features combined with the chemical composition may impart or deprive the resultant polymer, biocompatibility and resistance to biodegradation by the host tissue environment. These factors also influence other properties such as solubility and methods of processing and moulding.
Moreover, when a polymer is injected into the mammals, it normally, slowly disappears from the site of administration, however, this disappearance occurs in response to a chemical reaction such as hydrolysis, which normally is a part of biotransformation process and the said polymer is metabolised and eliminated from the body. This, however, sometimes leads to unnecessary metabolites, which cause untoward effects on various biological systems. Therefore, polymers, which are inert in/to the environment of use, and are eliminated or extracted intact from the site of administration as well as serve essentially as a rate limiting barrier to transport and release of a drug from it, may be of prime importance based on the intended functions. Again, the biodegradability of a polymer depends on the mechanical and chemical properties of the polymer. A variety of natural, synthetic, and biosynthetic polymers are bio- and environmentally degradable. A polymer based on the C—C backbone tends to be non-biodegradable, whereas hetero atom-containing polymer backbones confer biodegradability. Non-biodegradability/biodegradability can therefore be engineered into polymers by the judicious deletion/addition of chemical linkages such as anhydride, ester, or amide bonds, among others Common examples of non-biodegradable polymer materials include polyethylene vinyl acetate, polydimethyl siloxane, polyether urethane, ethyl cellulose, cellulose acetate, polyethylene and polyvinyl chloride.
There is a welter of reports available on the attempts made over the last few decades or so on development of nanoparticulate delivery systems for a large variety of drugs utilizing polymers. To name a few, these include the disclosures of:    i) Sakurai et al in U.S. Pat. No. 5,412,072, wherein a complex comprising a drug covalently bonded to a polymer composed of hydrophilic and hydrophobic fragments is found to render the said complex water soluble and thereby suitable for administration. The drugs utilized therein are in general less water soluble or insoluble compounds and the drug-polymer complex is reported to form polymeric micelles in aqueous solutions and becomes water-soluble high molecular polymerized drugs, useful and suitable for administration.    ii) Yokoyama et al in U.S. Pat. No. 5,449,513, wherein they report a polymeric micelle, which unlike that disclosed by Sakurai et al in U.S. Pat. No. 5,412,072 is not a complex wherein a drug is covalently bonded to a polymer, but rather one wherein the drug is entrapped within the polymer. The drugs utilized for entrapment are hydrophobic in nature. The polymeric micelle is in turn prepared by entrapment of hydrophobic drugs inside the polymeric shell through conventional methods such as ultra sonication, followed by purification of the micelles thus obtained through dialysis.    iii) Desai et al in U.S. Pat. No. 5,439,686; U.S. Pat. No. 5,362,478; U.S. Pat. No. 5,916,596; U.S. Pat. No. 6,096,331; U.S. Pat. No. 6,537,579 and U.S. Pat. No. 6,749,868, wherein polymeric micelle of substantially water-insoluble compounds are prepared. The water-insoluble compound is reported to be entrapped inside the polymeric shell to a significant extent and suitable for administration to a patient in need thereof either in a soluble or suspended form.
The polymers utilized by Sakurai et al in U.S. Pat. No. 5,412,072 are in general those comprising a hydrophilic segment selected from polyethylene glycol, polysaccharides, polyacrylamide etc and a hydrophobic segment selected from polyaspartic acid, polyglutamic acid, polylysine etc.
The polymers utilized by Yokoyama et al in U.S. Pat. No. 5,449,513 are in general those comprising a hydrophilic segment selected from polyethylene oxide, polymalic acid, polyaspartic acid, polyglutamic acid, polylysine, polysaccharide etc and a hydrophobic segment selected from poly(β-benzyl L-aspartate), poly(γ-benzyl L-glutamate), poly(β-substituted aspartate), poly(γ-substituted glutamate), poly(L-leucine), poly(L-valine), poly(L-phenylalanine), hydrophobic polyamino acids, polystyrene, polymethacrylate, polyacrylate, polymethacrylate amide, polyacrylate amide, polyamide, polyester, polyalkylene oxide and hydrophobic polyolefins.
The polymers utilized by Desai et al in U.S. Pat. No. 5,439,686; U.S. Pat. No. 5,362,478; U.S. Pat. No. 5,916,596; U.S. Pat. No. 6,096,331; U.S. Pat. No. 6,537,579 and U.S. Pat. No. 6,749,868 are in general those essentially bearing sulfhydryl groups or disulfide bonds within its structure e.g. Albumin (which contains 35 cysteine residues), Insulin (which contains 6 cysteine residues), Haemoglobin (which contains 6 cysteine residues per α2β2 unit), Lysozyme (which contains 8 cysteine residues), Immunoglobulins, α-2-Macroglobulin, Vitronectin, Vitronectin, Fibrinogen etc. Such polymers are substantially cross-linked through formation of disulphide bonds. Such polymers include both synthetic and natural polymers, which as mentioned hereinbefore, bear sulfhydryl groups or disulfide bonds within their structure. The sulfhydryl groups or disulfide linkages are reported to either be pre-existing or obtained through suitable chemical modifications. The natural polymers are reported to be preferred and include albumin proteins, oligopeptides, polynucleic acids etc.
However, the disadvantage with the polymeric micelles disclosed by Sakurai et al, Yokoyama et al, and Desai et al are that they all utilize polymers, both synthetic and natural, which are biodegradable. It might be mentioned that biodegradable polymers, although, are capable of influencing the drug release pattern as well as the release kinetics of the loaded drug, however, are not particularly preferred in drug delivery systems because they:    a) Have low plasma life time due to their rapid capture by the mononuclear phagocyte system (MPS) cells;    b) Lack response to physiological changes;    c) Lack consistent drug release kinetics which may economically and therapeutically cause waste of the drugs and other adverse effects; and    d) May cause increase of toxicity or immunogenicity since encapsulation of protein drugs involves organic solvents, which may cause protein denaturation.
Delivery systems wherein polymers, which are non-biodegradable, have been utilized and disclosed by:    i) Maitra et al in U.S. Pat. No. 5,874,111, wherein the drug is entrapped within the polymer resulting in a highly monodispersed polymeric hydrophilic nanoparticle being formed. The polymers utilized are those comprising of monomers like Vinylpyrrolidone (VP) or mixture of Vinylpyrrolidone and polyethyleneglycolfumarate (PEGF), etc.    ii) Maitra et al in U.S. Pat. No. 6,322,817, wherein the polymers utilized comprise of at least one type of an amphiphilic monomer selected from the group consisting of vinylpyrrolidone, acrylic acid, alkyl acrylates having a chain length of C3 to C6 functionalized polyethylene glycol of a molecular weight of 2000 to 6000, N-alkylacrylamide having a chain length of C3 to C6, and alkylcyanoacrylate having a chain length of C3 to C6. The drugs entrapped within the polymeric micelles are taxane derivatives, in particular Paclitaxel.    iii) Lowe et al in US 2005/0169882, wherein the polymers utilized comprise of a smart segment (which is non-biodegradable) and a biodegradable segment. More specifically, the smart, non-biodegradable segment comprises of poly(N-isopropylacrylamide), poly(N-alkylacrylamide), poly(N-n-propylacrylamide), poly(N-isopropylmethacrylamide), poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), elastin-like polypeptides, or a derivative thereof and the biodegradable segment comprises of polysaccharide, dextran, polyester, polylactide, poly(L-lactic acid), poly(D,L-lactic acid), poly(lactide-co-glycolides), biotinylated poly(ethylene glycol-block-lactic acid) etc.
In the case of polymers disclosed in U.S. Pat. No. 5,874,111, it might be noted that such polymers are prepared through polymerization of respective monomers and the polymeric material thus obtained is purified and isolated from an aqueous medium containing the same through a method of dialysis.
In the case of polymers disclosed in U.S. Pat. No. 6,322,817 also, the polymers are prepared through polymerization of respective monomers and is purified and isolated from an aqueous medium through a method of dialysis.
Similarly, the polymers disclosed by Lowe et al in US 2005/0169882 is also purified and isolated from an aqueous medium containing the same through a method of dialysis.
With regard to the compounds or drugs disclosed by Maitra et al in U.S. Pat. No. 5,874,111 for entrapment into polymers disclosed therein, they are primarily Antigens, Bovine serum etc. In the case of U.S. Pat. No. 6,322,817, the drugs for entrapment into polymers disclosed therein are primarily water-insoluble taxane derivatives, especially Paclitaxel, whereas Lowe et al in US 2005/0169882 disclose a wide host of drugs, which can be entrapped into the polymeric shell disclosed therein.
Further to the above, Burman et al in U.S. Pat. No. 6,365,191 teach a pharmaceutical composition comprising the polymer disclosed in U.S. Pat. No. 6,322,817, and more specifically a pharmaceutical composition of taxane derivatives, especially Paclitaxel. Herein, the said pharmaceutical composition is prepared by adding a solution of Paclitaxel in ethanol to an infusion vehicle comprising dextrose solution, to which has been added a solution of polymer in water containing an anionic surfactant and a buffering agent. Burman et al further claim that the pharmaceutical composition is stable for more than 12 hours without any precipitation of the drug from the perfusion fluid and that more than 90% (as analysed by HPLC) of the drug is entrapped within the polymeric micelle even after 24 hours of preparation of the perfusion fluid.
Even though, Burman et al in U.S. Pat. No. 6,365,191 claim that the pharmaceutical composition disclosed therein is in a nanoparticulate form, however, there is no mention in the specification as to the size of the claimed nanoparticulate form. The only mention about the particle size of the polymeric micelles containing Paclitaxel can be found in the disclosure by Maitra et al in U.S. Pat. No. 6,322,817, wherein the said nanoparticles containing Paclitaxel are reported to have a diameter in the range of 30-150 inn. Similarly, there is no mention about particle size of the polymeric micelles of the compositions disclosed by Lowe et al in US 2005/0169882.
It is important to note that the polymers disclosed in U.S. Pat. No. 5,874,111; U.S. Pat. No. 6,322,817; U.S. Pat. No. 6,365,191 and US 2005/0169882 are prepared from one or more monomers, which include Vinylpyrrolidone and N-Isopropylacrylamide. It is further, important to note that Vinylpyrrolidone and N-Isopropylacrylamide are toxic compounds, whose presence in a pharmaceutical composition meant for human/animal consumption is not only frowned upon by Health Authorities worldover, but also comes under stringent quality adherence, with strict limits set by Pharmacopoeial Forums worldover. For instance, the level of monomeric Vinylpyrrolidone in the polymer, Polyvinylpyrrolidone, as well as any other polymer containing Vinylpyrrolidone as a monomer, as prescribed by US and European Pharmacopoeias should not exceed a limit of 0.001% (i.e. <10 ppm).
There is a grave danger that in the methods described in U.S. Pat. No. 5,874,111; U.S. Pat. No. 6,322,817; U.S. Pat. No. 6,365,191 and US 2005/0169882 for preparation and isolation of polymers utilizing Vinylpyrrolidone as one of the monomers, the said monomer i.e. Vinylpyrrolidone may be present in limits higher than 0.001% (i.e. >10 ppm).
There is an equally grave danger that pharmaceutical compositions containing such polymers, utilising Vinylpyrrolidone as one of the monomers may also contain Vinylpyrrolidone as a monomeric contaminant and that the said monomer i.e. Vinylpyrrolidone may also be present in limits higher than 0.001% (i.e. >10 ppm).
This could be true especially with regard to the pharmaceutical compositions disclosed in U.S. Pat. No. 6,322,817; U.S. Pat. No. 6,365,191 and US 2005/0169882.
It need not be overemphasised that any chemical reaction including a reaction for preparation of polymers is never complete in the sense that invariably one or more of the reactants are left over in the product. This would apply to polymerization reactions involving Vinylpyrrolidone as a monomer and depending on the molar or weight proportions of Vinylpyrrolidone utilized, there is every possibility that some amount of the Vinylpyrrolidone would remain as a contaminant in the polymeric products prepared thereof.
In connection with the above, the present inventors concerns were found true, wherein analysis of the polymer prepared by polymerization of three monomers, viz. Vinylpyrrolidone (VP), N-isopropylacrylamide (NIPAM) and Ester of Maleic anhydride and polyethylene glycol (MPEG) exactly as per the description given in Examples I, II and III of U.S. Pat. No. 6,322,817 were found to contain an amount of N-isopropylacrylamide (NIPAM) and Vinylpyrrolidone (VP) as summarized Table-I.
TABLE IAmount of Residual Monomers In The Polymer PreparedAs Per The Method Described In Examples I, IIand III Of U.S. Pat. No. 6,322,817Monomer% w/w Detected In The PolymerNIPAM0.066-0.076 (660-760 ppm)VP0.008-0.011 (80-110 ppm) 
It would be abundantly evident that the amount of Vinylpyrrolidone found in the polymer is at least more than eight times the toxic limit of 0.001% (i.e. 10 ppm), wherein the pharmaceutical composition containing such a polymer would be very unsafe and highly toxic for administration to humans or animals.
A need, if not imperative exists not only for a polymer substantially free of toxic monomers, such as N-isopropylacrylamide (NIPAM) and Vinylpyrrolidone (VP) but also for pharmaceutical compositions comprising a polymer, which are substantially free of toxic monomeric contaminants, such as N-isopropylacrylamide (NIPAM) and Vinylpyrrolidone (VP).
It might also be noted that the pharmaceutical compositions disclosed by Burman et al in U.S. Pat. No. 6,365,191 are reported to have a stability of only about 12 hrs or more with 90% or more of the drug entrapped within the polymeric micelle at the end of 24 hours. Further, the pharmaceutical compositions disclosed by Lowe et al in US 2005/0169882 are reported to have a loading of the biologically active substances of approximately 40% only, with a release of the biologically active substance between a few hours to up to several days.
A further need exists for pharmaceutical compositions, which have longer stability as well as higher drug loading, which moreover are safe and less-toxic.
The present invention is a step forward in advancement of not only providing a polymer, which is substantially free of toxic monomeric contaminants, but also providing a pharmaceutical composition comprising such a desired polymer, which is safe for human/animal administration and has longer stability.